The mean rock volume percent was determined for each of 11 standard
layers for each map unit of each state using data from the STATSGO
Comp and Layer tables. The standard layers were introduced because
of the wide variation in the number, thickness, and depth to top
and bottom of soil layers in the STATSGO data from one soil
component to another, even within the same map unit.

Determining the volumetric percent of rocks for the 11 standard layers
required three main steps:

Computing the percent of rock by volume in each component
layer.

For each component, determining the contribution of each component
layer to the 11 standard layers.

For each map unit, combining the contributions of all components to
compute the mean rock volume percent for each standard layer.

The computation of the rock volume in each component layer uses four
pairs of variables in the STATSGO Layer table: the bulk density
(BDL/BDH); the fraction by mass of the soil material less than 3 inches
in diameter which passes a No. 10 (2 mm) sieve (NO10L/NO10H); the
fraction by mass composed of rocks with sizes between 3 and 10 inches
(INCH3L/INCH3H); and the fraction by mass composed of rocks larger than
10 inches (INCH10L/INCH10H). For each pair of variables, the "L" and
"H" values give the upper and lower end of the range of values for the
quantity within the layer.

Since the size variables are given in terms of percent by mass,
computation of the volumetric rock fraction requires combining the mean
bulk density (BD) of the fine soil component with the mean particle
density (PD) of the rocks. In terms of the variables recorded in the
STATSGO Layer table, the mean rock volume RVOL for a layer is given by

RVOL = 100 - FINES / DENOM

where the mass percent of fines soil is given by

FINES = NO10 * (100 - INCH3 - INCH10)

and the denominator is

DENOM = FINES + (100 - FINES) * (BD / PD)

The values of each of the variables BD, NO10, INCH3, and INCH10 were
computed as the arithmetic mean of the "L" and "H" ends of their ranges
as specified in the Layer table. PD was assigned a value of 2.65
g/cm³, the mean density of silicate rocks.

The contributions of each component layer to the standard layers for a
given map unit were determined using the component layer depths
specified by Layer table variables LAYDEPL and LAYDEPH, the mean depth
to bedrock for each component calculated by averaging Comp table
variables ROCKDEPL and ROCKDEPH, and the percent of the area of the map
unit covered by each component as specified by COMPPCT. For each
component, the layers defined in the Layer table were compared with each
standard layer in turn. If the standard layer was entirely included
within one of the component layers, the rock volume value for the layer
was multiplied by the COMPPCT value to determine the weighted
contribution of the component to the standard layer. If the standard
layer overlapped two or more component layers, the rock volume values
for each component layer were first weighted in proportion to the amount
of overlap before multiplication by the COMPPCT value. The region from
the bottom of the last component layer to the bottom of the last
standard layer, if any, was assumed to be the same as the lowest
component layer down to the mean bedrock depth, below which the rock
volume percent was set to 100.

The weighted contributions of all components to each standard layer were
then summed to obtain the mean rock volume values for the map unit.
However, if a component was identified as all water (COMPNAME = "WATER")
or if the Layer table records contained contradictory values for
particle size and density (see below), the component was omitted from
the computation. If the map unit was entirely water, it was assigned a
rock volume of zero. Otherwise, if all non-water components were
unusable because of invalid or contradictory information, the map unit
was assigned a flag value of 101%.

Two major problems were encountered which affect the validity of the
computed values.

Many components specify ROCKDEPL = ROCKDEPH = 60 inches (152 cm) to
infer that the soil was not examined below this depth. In most cases,
bedrock is not actually present. However, there was no way to determine
whether this was the case for any given component. Accordingly,
the rock volume values for layers extending below this depth (the
deepest two standard layers) will frequently be misleading.

A number of non-water components, and in some cases entire map
units, have BDL = BDH = 0 for all component layers, even though other
variables for the layer (e.g., texture) contradict the BD values. In
addition, values for NO10, INCH3, and INCH10 were often missing, and in
some cases contradictory. In particular, there were about 250
components for which INCH3L + INCH10L exceeded 100%, suggesting that
when the values were entered the INCH3 variable was incorrectly
interpreted as the total percent by mass of rock fragments larger than 3
inches, rather than being only the fraction between 3 and 10 inches as
specified in the STATSGO variable definitions. It was not possible to
determine for how many additional components, with INCH3L + INCH10L <
100%, this same incorrect interpretation was made. In all cases, an
attempt was made to compute the rock volume using the values as
specified, and setting missing NO10, INCH3, and INCH10 values to zero.
In a number of cases in which the Layer table specified BDL = BDH = 0,
the top of the layer (LAYDEPL) was at or below the mean depth to
bedrock; for these cases the layer was assumed to be bedrock, and the
rock volume was set to 100%. A component was omitted from the
computations for a map unit only if the given data values led to
nonsensical results, such as a negative value for FINES or a zero or
negative value for DENOM.